US Defence Secretary Leon Panetta is in Afghanistan for talks as the US seeks to contain fallout from the massacre of 16 Afghan villagers by a US soldier.
Officials say the trip was planned before Sunday's killings, which Mr Panetta described along with other recent events as "deeply troubling".
Mr Panetta is due to have talks with President Hamid Karzai, provincial leaders and senior officials.
Separately, two bomb attacks in the south left at least nine Afghans dead.
Correspondents say Mr Panetta's unnannounced visit has taken on a new meaning as political pressure mounts on Afghan and US officials over the unpopular war.
The defence secretary addressed US troops as fears mount that they could be the victims of a Taliban-led backlash over the killings of the villagers, including nine children, by the rogue US soldier.
His arrival at Bastion Airfield in Helmand province came a day after protests over the massacre flared in the eastern city of Jalalabad.
Mr Panetta is the most senior US official to visit Afghanistan since the shootings.
He told reporters ahead of his trip that he believed that American strategy in Afghanistan was working and would withstand repercussions from the killings.
"As tragic as these acts of violence have been, they do not define the relationship between the coalition and Afghan forces, and the Afghan people," Mr Panetta told troops in southern Afghanistan after arriving.
Earlier, he said that the US was "on the right path" and that "despite these kinds of events" US strategy in Afghanistan should not be undermined.
"It's important that we push on, and that we bring this war to a responsible end and achieve the mission that all of us are embarked on."
His two-day trip coincides with discussions in the US between UK Prime Minster David Cameron and President Barack Obama over a possible "end game" to the war in Afghanistan.
The leaders are expected to agree that Afghan forces should take over a lead combat role by mid-2013 - earlier than planned - but the US president has said there will be no "rush for the exits".
As Mr Panetta arrived in Afghanistan, a motorcycle bomb blast in Kandahar city killed at least one Afghan intelligence officer and wounded three others.
In neighbouring Helmand province, a roadside bomb attack in Marjah district killed eight civilians travelling in a minibus, a spokesman for the provincial governor told the BBC.
He said that the Taliban were behind the attack, which took place less than 1km from a government guest house where senior members of an Afghan delegation tasked to investigate Sunday's killings were staying.
The soldier who allegedly carried out the attack was caught on surveillance video that showed him approaching his base, laying down his weapon and raising his arms in surrender, an Afghan official who viewed the footage told the AP news agency.
The official said the Americans had showed the footage to Afghan authorities to prove that only one person was involved in the shootings.
Correspondents say that any major discrepancy between the official US and Afghan accounts of the killings is likely to deepen distrust between them, with relations already strained even before the latest violence.
Last month there was outrage over US troops inadvertently burning Korans on an American base. In the protests that followed, at least 30 Afghans and six US military personnel were killed.
Mr Panetta told troops that the burning of Korans, Sunday's shootings and attacks on coalition troops were "deeply troubling".
There was also fury in Afghanistan following the posting of a video on the internet in January apparently showing four US marines urinating on Taliban corpses.